Introduce delay in setting users offline.

This is part of an effort to reduce actions in light of https://github.com/cargomedia/CM/issues/1444. The aim is to get rid of online/offline (maybe also visible/invisible) actions caused by refreshing a page (closing and opening new websocket).
Also solves https://github.com/cargomedia/sk/issues/601

Idea: Add offlineStamp to cm_user_online, check regularly and only really set offline 10 seconds after that stamp.